Method for forming a chemical coating on aluminum or aluminum alloy

ABSTRACT

A METHOD FOR FORMING A CHEMICAL COATING ON ALUMINUM OR ALUMINUM ALLOY IN WHICH THE ALUMINUM OR ALUMINUM ALLOY IS IMMERSED IN AN AQUEOUS SOLUTION CONTAINING MONOETHYLAMINE OR DIETHYLAMINE. THE AQUEOUS SOLUTION CAN BE HEATED EITHER BEFORE OR AFTER THE ADDITION OF MONOETHYLAMINE OR DIETHYLAMINE. AFTER IMMERSING, THE METAL MAY BE SUBJECTED TO PRESSURIZED STEAM.

United States Patent Ofice 3,728,164 METHOD FOR FORMING A CHEMICAL COATING ON ALUMINUM OR ALUMINUM ALLOY Takashi Abe, Koji Aizawa, and Masanobu Oguri, Osaka, Japan, assignors to Showa Aluminium Kabushiki Kaisha, Osaka, Japan No Drawing. Filed Sept. 23, 1970, Ser. No. 74,837 Claims priority, application Japan, Oct. 13, 1969, 44/ 82,122 Int. Cl. C23f 7/06 US. Cl. 148-637 1 Claim ABSTRACT OF THE DISCLOSURE A method for forming a chemical coating on aluminum or aluminum alloy in which the aluminum or aluminum alloy is immersed in an aqueous solution containing monoethylamine or diethylamine. The aqueous solution can be heated either before or after the addition of monoethylamine or diethylamine. After immersing, the metal may be subjected to pressurized steam.

BACKGROUND AND SUMMARY The present invention relates to a method for forming a chemical coating on aluminum or aluminum alloy (hereinafter referred to as aluminum). The coating so formed contains boehmite, i.e., hard, crystalline water-insoluble AlOOH.

Since aluminum has come to be employed for versatile uses in recent years, there arises a need to subject aluminum to some surface treatment to enhance durability. Accordingly, various methods of surface treatment such as anodic oxidation processes for providing electrochemical coating, or MBV process, Alodine process, etc. for forming chemical coating are used depending upon the purposes. However, anodic oxidation process has drawbacks in that it requires complex electrical equipments and in that distribution of electricity is not necessarily effected uniformly depending upon the shape of articles with the result that some portions are left uncoated even after treatment. On the other hand, MBV and Alodine processes, though generally used, are not satisfactory because the coating is poor in corrosion resistance. Also known in recent years is the boehmite process in which aluminum is immersed in a boiling aqueous solution containing triethanolamine, monoethanolamine or the like to form a chemical coating, but the coating thus formed is generally thin and therefore it is desired to improve its corrosion resistance.

In view of the above, the present invention contemplates providing a uniform and corrosion-resistant coating without using a complex apparatus, the invention being characterized in that aluminum is treated by being immersed in a boiling aqueous solution containing at least one member selected from the class of monoethylamine and diethylamine.

DETAILED DESCRIPTION OF THE INVENTION In more detailed description, the present invention is accomplished by immersing aluminum for several tens of minutes in a solution in boiling or resembling state prepared by adding monoethylamine, diethylamine or both amines to pure water or distilled water. The water may be heated to boiling before or after the addition of the amine.

The amount of the amine to be added at this time may preferably be about 0.5 cc./liter, to 30 cc./liter. If the amount is smaller than 0.5 cc./liter, satisfactory corrosion resistance will not be obtained, while the addition of more than 30 cc./liter results in hardly any noticeable effect but rather expedites deterioration of the treating solution.

3,728,164 Patented Apr. 17, 1973 Further prior to the treatment of the present invention, an ordinary pretreatment such as washing or removal of oily substances on the surface of aluminum is performed, while the present treatment is followed by an aftertreatment employing pressurized steam or the like.

Examples of the present invention will be given below.

EXAMPLE 1 In order to remove oil and fat and oxidized film from the surface of aluminum for cleaning, aluminum was immersed for 2 minutes in a 5% solution of caustic soda heated to C. and then washed with water. The aluminum was thereafter immersed for 30 seconds in a 30% nitric acid solution for neutralization and Washed with water. After this pretreatment, 1100 aluminum plates (1 mm. x 56 mm. x mm.) obtained were subjected to a treatment in accordance with the present invention by immersing them for 30 minutes and 60 minutes respectively in a boiling aqueous solution which was prepared by adding 3 cc./liter of monoethylamine to pure water. Aftertreatment was conducted for 20 minutes in a pressurized steam of 4 kg./cm. The number designation herein used refers to the Aluminum Association Standard Number for wrought aluminum alloy.

The same treatments were carried out in the same mannet as above except that diethylamine, and both amines of monoethylamine and diethylamine were used in place of monoethylamine. The ratio of monoethylamine and diethylamine was about 1:1.

The aluminum plates chemically treated in the abovementioned manner were tested to measure the weight of the coating and resistance to alkali. and to acid. The results are stated below.

As reference examples the results obtained by using monoethanolamine, triethanolamine, triethylamine and triallylamine are also given. The treating conditions in these cases were exactly the same as those in the case of monoethylamine.

(1) Measurement of weight of coating The weight of the coating was measured to observe the state of the coating formed.

From the results of the measurement it will be seen that chemical treatments employing monoethylamine, diethylamine both amines of monoethylamine and diethylamine and monoethanolamine resulted in coatings of considerable thicknesses.

(2) Alkali resistance test Aluminum specimens treated as described above were immersed in a 0.1 N solution of caustic soda heated to 35 C. and the difference between the weight after lapse of a given period of time and the weight before the test was measured to observe the alkali resistance of the specimens. The test of aluminum specimens chemically treated by using both amines of monoethylamine and diethylamine was omitted.

Weight of Treating coating Additives time (min.) (mg./dm.

Present invention:

Monoethylamine '28 23:2 Diethylamine--;. Reference examples:

Mouoethanolamine-;;;-;.;..{ 28 g: g 2;: Triethanolamineg g Triethylamine...;' 2g fa; V v 30 41. 7 1, 573. 9 Tnanylamme From the test results it will be noted that high resistance to alkali was efiected by chemical treatments which employed monoethylamine, diethylamine and monoethanolamine.

(3) Acid resistance test The specimens were immersed in a. 0.5 N solution of hydrochloric acid heated to 35 C. and the dilference between the weight after lapse of a given period of time and the weight before the test was measured to observe the acid resistance of the specimens. The test of aluminum specimens chemically treated by using both amines of monoethylamine and diethylamine was omitted.

The test results show that superior resistance to acid was obtained by chemical treatments employing monoethylamine and diethylamine.

As apparent from the results of the foregoing tests the specimens chemically treated in accordance with the present invention have thick coatings which are excellent in resistance to alkali and to acid.

EXAMPLE 2 Pretreatment and aftertreatment were conducted under the same conditions as in Example 1. In carrying out treatments in accordance with the present invention, monoethylamine, diethylamine and both amines of monoethylamine and diethylamine were added in d'ifierent amounts of 0.5, 3, 12, 24, 30 cc./1iter. The weight of coating and resistance to alkali and to acid were measured with respect to the specimens thus treated. The results are given below.

As reference examples, treatments were conducted using triethanolamine which is commonly used. The results are also shown.

(1) Measurement of weight of coating After the formation of chemical coating, the specimens were immersed in a boiling stripping solution of phosphoric acid-chromic acid. Based upon the difference of weight produced by the immersion, the weight of coating was measured to observe the state of the coating formed.

Amount Treating Weight of adde time coating Additives (cc.lliter) (min.) (mgJdmfl) Present invention:

5 30. 2 10 33. 8 0. 5 15 37. 5 3O 38. 4 6O 39. 5 5 39. 9 10 43. 3 3 15 48. 2 30 57. 0 60 58. 4 5 54. 7 10 as. 6 Monoethylamme.....; 12 15 83. 3 30 104. 2 60 124. O 5 68. 5 10 84. 2 24 15 101. 6 30 128. 2 60 155. 0 5 79. 4 10 96. 8 30 15 116. 8 30 147. 7 60 178. 2 5 21. 0 10 21. 5 0. 5 15 22. 9 30 23. 2 60 24. 3 5 29. 5 1O 31. 0 3 15 32. 4 30 33. 4 60 35. 2 5 44. 1 Diethylamine 12 30 58. 0 60 61. 0 5 54. 2 10 63. 5 24 15 70. 0 30 82. 0 60 85. 7 5 67. 3 10 75. 5 30 15 85. 7 l 30 90. 6 6 97. 1 5 28. 0 10 31. 7 0. 5 15 36. l 30 36. 2 60 36. 3 5 39. 1 10 40. 6 3 15 44. 7 30 50. 4 6g 52. 0 51. 1 Monoethylamine and 10 54. 3 dwthyla-miue 12 15 65. 0 30 83. 7 60 103. 0 5 52. 7 10 70. 3 24 15 88. 0 30 114. 4 60 140. 0 5 66. 5 10 83. 8 30 15 91. 9 28 132. 4 Reference example: 60. 2 5 9. 2 10 10. 4 0. 5 15 12. 8 30 13. 9 60 14. 8 5 15. 9 10 17. 1 3 15 19. 8 30 20. 6 60 21. 5 1 5 O. Triethauolamine.. .;;.21.. 12 15 32. 5 30 33. 7 60 36. 8 5 32. 2 10 35. 7 24 15 36. 5 30 38. 5 60 38. 8 5 34. 9 10 35. 9 30 15 36. 9 30 38. 6 60 39. 1

From the results of the measurement it will be seen that the chemical treatments of the present invention employing monoethylamine, diethylamine and both amines tic soda heated to 35 C. and the difference between the weight after lapse of a given period of time and the weight before the test was measured to observe the alkali resistance of the specimens.

(2) Alkali resistance test Specimens were immersed in a 0.5 N solution of caus- 5 m n 0 H h z. 3 m

g S a m m h H 2 m at m m T mw wmm m m A k S e w m d A Present invention:

82150165716416665393642 2 ZQQLLLzmKLL mm umm mw s2 42 i 9 1 5 5 0 00 0 0 0 0 wlme mmae imam mmwe mmsm llv llll lll 3 m M m fl fallll Monoethylamine and diethylamine 8 1 TABLE-Continued Amount Treating Weight reduction (mgJdmfi) ini added time Additlves (ee.[liter) (min.) 1 hour 2 hours 3 hours Reference example:

30 216.5 60 166.5 5 1,344.2 10 185.4 Triethnnnlam nn 12 15 37. 3 30 17.0 60 8.4

It W111 be apparent from the test results that the (3) ACld resistance test 33 :8531 gi gi g ggg ig g pz g gggg 30 The specimens were lmrnersed 1n a 0.5 N solution of y 1 y 1 e a 0 mm hydrochloric acid heated to C. and the difference beethylarnine and diethylamine produced coatings which were higher in alkali resistance than those obtained by ggg i s i g g g i ig i g gg gg lg 32 2 2:

methanolammethe acid resistance of the specimens.

Amount Treaiting Weight reduction (mgJdmJ) inlme added Addifives -l (min.) 4 hour 8 hours 14 hours Present invention:

5 13. 5 10 12. 4 6 0. 5 15 10. 9 270. 9 U. 3O 7. 0 220. 4 6. 9 158. 3 5 7. 1 81. 8 1O 6. 8 70. 4 3 15 4. 2 27. 9 972. 5 30 2. 7 11. 7 329. 7 60 2. 6 9. 8 89. 6 5 4. 9 22. 7 72. 0 10 4. 2 14. 3 49. 2 Monoethylalmne.;1.:::;t::';1'::::;:::; 12 15 2. 3 8. 1 26. 3 30 2. 1 6. 4 23. 4 60 2. 0 6. 3 21. 5 5 3. 7 12. 9 36. 3 10 3. 2 10. 8 29. 8 24 15 2. 2 7. 9 26. 1 30 1. 8 6. 1 23. 2 69 1. 7 6. 0 21. 0 5 4. 2 14. 9 38. 8 10 3. 9 13. 1 31. 6 30 15 2. 8 9. 7 25. 9 30 2. 6 8. 2 23. 3 60 2. 1 7. 8 23 1 5 16. 9 652. 3 1O 14. 2 548. 1 O. 5 15 12. 8 436. 2 30 11. 8 425. 0 60 10. 9 351. 8 5 11. 1 325. 3 10 19. 6 322. 5 3 15 9. 8 129. 7

3O 7. 6 111. 8 60 6.2 93.8 5 3. 8 16. 5 698. 8 10 3. 7 9. 6 232. 3 Diethylamine 12 15 2. 9 6. 3 56. 8 30 2. 4 6. 1 30. 8 60 2. O 5. 9 21. 7 5 1. 5 9. 7 28. 4 10 1. 5 9. 3 22. 9 24 15 1. 2 5. 9 18. 6 3O 0. 7 5. 1 14. 9 60 0. 7 5. 9 12. 7 5 4. 3 14. 4 33 3 10 4. 1 14. 2 32. 9 30 15 3. 2 7. 5 17. 4 30 3. 2 6. 6 15. 5 60 3. O 6. 2 14. 4

TABLE-Continued Amount Treating Weight reduction (mgJdmfl) inadded time Additiv (co/liter) (min.) 4 hour 8 hours 14 hours 30 4. 3 79. 1 720. 1 60 4. 1 60. 8 431. 7 5 4. 6 10. 8 57. G 10 4. 1 9. 2 47. Monoethylamine and dietylamme 12 15 3. 9 8. 1 28. 8 30 3. 7 7. 4 16. 3 60 3. 2 7. 2 15. 0 4. 2 9. 1 45. 7 4.0 8.5 40. 2 24 3. 8 8. 0 24. 9 3. 5 7. 1 15. 7 60 3. O 6. 7 13. 3 5 7. 4 10. 7 72. 6 10 7. 2 9. 8 39. 7 30 15 7. 2 9. 1 30. 3 30 7. 0 8. 2 22. 6 60 6. 8 7. 7 20. 4 Reference example: F

. 5 364.6 10 277. 5 0. 5 15 145. 6 30 75. 8 60 30. 5 5 44. 6 10 38. 7 3 15 13. 8 3O 7. 8 60 3. 9 5 4. 6 10 4. 4 Triethanolarnine 12 15 4. 7 30 10. 6 60 16. 4 5 5. 3 10 5. 6 24 15 15. 9 30 18. 5 60 26. 0 5 8. 7 10 10. 4 30 15 20. 0 30 22. 7 60 30. 3

It will be noted from the test results that the chemical coatings formed in accordance with the present invention have higher resistance to acid than those formed by using triethanolamine.

As apparent from the results of the respective tests stated above, the specimens subjected to the chemical treatment of the present invention are more excellent than those obtained by using triethanolamine with respect to the state of the coating formed and resistance to alkali and to acid.

While in the foregoing specification, a detailed description of a specific embodiment of our invention was set forth for the purpose of illustration, it is to be understood that many of the details herein given may be varied considerably by those skilled in the art without departing from the spirit and scope of our invention.

We claim:

1. A method for forming a boehmite-containing chem ical coating on a metal selected from the class of aluminum and aluminum alloy comprising the steps of im- References Cited UNITED STATES PATENTS 3,380,860 4/1963 Lipinski 1486.27

3,544,391 12/1970 Scott l48--6.27

FOREIGN PATENTS 719,680 '10/1965 Canada 148-627 RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 148-63 

